Method and apparatus for suppressing corrosion of carbon steel, method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant, and film formation apparatus

ABSTRACT

The present invention is a method for suppressing corrosion of carbon steel members composing a nuclear power plant. That is, the processing solution contains a chemical including iron (II) ions, an oxidizing agent for oxidizing at least one part of the iron (II) ions into iron (III) ion, and a pH adjustment agent for adjusting pH. The pH of the processing solution is adjusted in the range of 5.5 to 9.0 by the pH adjustment agent. The processing solution is introduced into a purifying system pipe having the carbon steel members. The iron (II) ions are adsorbed on an inner surface of the purifying system pipe, namely, a surface of the carbon steel members. The ferrite film is formed on the surface of the carbon steel members by oxidizing the absorbed iron (II) ions. Therefore, corrosion of the carbon steel members is suppressed by the ferrite film.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for suppressing corrosion of carbon steel, method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant, and film formation apparatus.

Well-known nuclear power generation plants are, for example, boiling water type power generation plant (hereinafter referred to as BWR plant) and pressurized water type power generation plant (hereinafter referred to as PWR plant). The BWR plant supplies feed water into with a reactor having a reactor pressure vessel in which a plurality of fuel assemblies are loaded, through a feed water system, and causes cooling water (coolant) to boil into steam. Almost all of the generated steam is supplied to a steam turbine to drive the steam turbine and a power generator connected to the turbine in order to generate electric power. The steam exhausted from the steam turbine is condensed into water by a condenser. The condensed water is returned as feed water to the reactor by the feed water system. The PWR plant introduces high-temperature high-pressure cooling water heated in the reactor into a steam generator and heats feed water being fed to the steam generator into steam by the high-temperature high-pressure cooling water. The steam is supplied to the steam turbine. The low-temperature cooling water coming from the steam generator is returned to the reactor. The PWR plant has a secondary system including a steam generator, steam pipes, feed water pipes, and so on.

In nuclear power plants such as BWR plant and PWR plant, wetted surface, which contact with the cooling water, of major apparatus such as a reactor pressure vessel are usually made of stainless steel and nickel-based alloy to protect the wetted surface against corrosions. However, major structure members of the feed water and condensate systems mainly use carbon steel members to reduce the plant construction cost and avoid stress corrosion cracking of stainless steel to be caused by high-temperature water that flows through the feed water and condensate systems.

However, the carbon steel members that constitute the feed water and condensate systems also contain surfaces that are wet with the cooling water and cannot be free from being corroded when wet. In the BWR plant arranging the carbon steel members in the downstream of a purifying apparatus, corrosive products of the carbon steel members may flow into the reactor and become activated corrosive products there. Further, the corrosive products may cause reduction in heat exchange efficiency of the secondary system of the PWR plant.

To suppress corrosion of carbon steel members that constitute the power plant, there have been proposed, for example, a method for forming an oxide film on the surface of the carbon steel members by feeding oxygen into the feed water system of the plant and a method for keeping feed water alkaline (pH of 7 or greater) by feeding ammonia, hydrazine, or other chemicals into the feed water system of the plant (for example, Japanese Patent Laid-open No. 2000-292589).

In addition to the above consideration, metallic impurities that slightly generate in the reactor water (that is the cooling water in the reactor pressure vessel) are also removed positively by purifying part of the reactor water in a reactor water purifying apparatus. However, in spite of the above-mentioned corrosion suppressing measures, traces of metallic impurities inevitably exist in the reactor water and part of the metallic impurities deposit as metal oxide on the surfaces of fuel rods in fuel assemblies. Metal elements on the surfaces of the fuel rods cause nucleus reaction by irradiation of neutrons emitted from nuclear fuel in the fuel rods and produce radionuclide such as cobalt 60, cobalt 58, chromium 51, manganese 54, and so on. Almost all of these radionuclide remain deposited as oxides on the surfaces of the fuel rods. However, part of the radionuclide dissolves into the reactor water under a specific solubility of respective oxides that contains the radionuclide and finally become insoluble solids called crude in the reactor water. While circulating in the primary cooling system together with the reactor water, the radionuclide in the reactor water deposit on the wetted surfaces of structure members such as stainless steel and Inconel and carbon steel members of the reactor water purifying system pipes. As the result, persons who are working on periodic inspection of the nuclear power generation plant are possibly exposed to radiations from the surfaces of such carbon steel members. Particularly, an advanced BWR plant has no re-circulation pipe. Therefore, the carbon steel pipes of the reactor water purifying system and the residual heat removal system, and the like greatly affects to atmosphere dose in the reactor containment vessel. The exposure dose during working is controlled to be under the specified value for each worker. Lately, however, this specified value has been reduced and the exposure dose of each person must be reduced as low as economically possible.

So, various methods such as a method of reducing deposition of radionuclide on the inner surface of carbon steel pipes and a method of reducing the concentration of radionuclide in the reactor water, etc. are studied. For example, Japanese Patent Laid-open No. Sho 58 (1983)-79196 discloses a method of suppressing to take radionuclide such as cobalt 60 and cobalt 58, etc. into oxide films by injecting metal ions such as zinc into the reactor water and forming a close oxide film including zinc on the surface, on which the reactor water is contacted, of the re-circulation system pipes. Further, Japanese Patent Laid-open No. Hei 9 (1997)-166694 discloses a method of making the reactor water alkaline (pH of 7 or greater) before radionuclide are dissolved or released into the cooling water and forming oxide films on the inner surfaces of the re-circulation system pipes and the reactor water purifying system pipes through which the reactor water flows during the operation of the plant.

SUMMARY OF THE INVENTION

However, a conventional method of injecting oxygen to feed water of the power generation plant cannot suppress corrosion of metals when oxygen injection is stopped and must keep on injecting oxygen during the operation of the power generation plant. This method runs contrary to the recent plant tendency that keeps the in-reactor environment in the reducing status in order to suppress stress corrosion cracking of the stainless steel structure members.

Similarly, another conventional method (e.g. Japanese Patent Laid-open No. 2000-292589) of adding chemicals to feed water of the nuclear power plant to control pH of the reactor water to greater than 7 is forced to keep on feeding chemicals during the operation of the nuclear power plant. Further, since the added chemicals increase the load of the condensate purifying apparatus, the radioactive wastes from the condensate purifying apparatus may increase. Accordingly, it is desired to suppress corrosion of carbon steel members that constitute the nuclear power plant.

The method of Japanese Patent Laid-open No. Sho 58 (1983)-79196 that injects the metal ions such as zinc into the reactor water has problems that injection of zinc ions must be always continued to the reactor water during the operation of the nuclear power plant, that depleted zinc must be used to avoid zinc itself being activated, and that these requirements push up the power generation cost.

The method disclosed in Japanese Patent Laid-open No. Hei 9 (1997)-166694 for forming oxide films forms oxide films on surfaces of the structure members in the operating temperature range (250° C. to 300° C.) of the BWR. This method cannot be formed oxide films on low-temperature surfaces of the reactor water purifying system and the carbon steel pipes of the residual heat removal system except for the high-temperature surface of the reactor water purifying system. Further, just after the nuclear power plant starts to operate for example, after chemical decontamination, radionuclide are inevitably contained also in the reactor water being used for formation of oxide films. Accordingly, it is desired to suppress deposit of radionuclide onto the surface of carbon steel members.

A first object of the present invention is to provide a method of suppressing corrosion of carbon steel members that constitute the plant and a corrosion suppressing apparatus thereof.

A second object of the present invention is to provide method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant (e.g., carbon steel members of the reactor water purifying system and the residual heat removal system) and film formation apparatus.

To attain the first object of the present invention, inventors of the present invention made various studies and researches and found that corrosion of the wetted surfaces of carbon steel members can be effectively suppressed by forming a fine film of ferrite (for example magnetite or nickel ferrite) of low water-solubility on the surface of the carbon steel members. The ferrite film functions as a protective film that prevents the carbon steel member from being wet with water.

Referring to FIG. 1, the effect of suppressing corrosion of carbon steel members by using the result of an experiment which attains the first object of the present invention will be explained. The vertical axis of FIG. 1 indicates the relative weight reduction of samples A, B, and C. The sample A is a carbon steel piece whose surface is mechanically polished. The sample B is a carbon steel piece whose surface is covered with a magnetite film. The sample C is a carbon steel piece whose surface is covered with a nickel ferrite film. The inventors kept these samples (A, B, and C) under pure water at ordinary temperature for ten days and then measured their weight reduction.

As shown in FIG. 1, the sample B forming a magnetite film and the sample C forming a nickel ferrite film produce less weight reduction than the sample A. In other words, the samples B and C are protected better than the sample A against corrosion. The corrosion suppressing effect of the sample B is less than that of the sample C because some parts of the film of the sample B are not so resistant to corrosion and because corrosion starts from there and destroys the nearby film.

A well-known technology to form ferrite film on magnetic recording media (see JP63-15990B) can be used to form ferrite films on the surfaces of major components of the power plant. However, since this technology (JP63-15990B) uses chlorine to form ferrite films, chlorine must be avoided when this technology is applied to the nuclear power plant to form ferrite films for assurance of the soundness (e.g., resistance to corrosion) of structure members of the nuclear power plant. Therefore, the technology of the present invention is different from the technology disclosed in JP63-15990B.

To attain the above first object, the present invention is characterized by a method of using non-chlorine chemicals, adsorbing iron (II) ions on the surface of carbon steel members composing a nuclear power plant, oxidizing the adsorbed iron (II) ions to form a ferrite film under a temperature condition from ordinary temperature to 200° C., preferably from ordinary temperature to 100° C., more preferably from 60 to 100° C., and thus protecting the carbon steel members by the ferrite film against corrosion.

In detail, this method adds a chemical containing iron (II) ion in organic acid or carbonic acid, an oxidizing agent that oxidizes iron (II) ions into iron (III) ion, and a pH adjustment agent that adjusts pH of the solution in the range of 5.5 to 9.0 to the processing solution. When this processing solution touches the surface of carbon steel members, a ferrite film is formed on the surface.

Particularly, chemicals including iron (II) ions should preferably be organic acid that can be easily decomposed after film formation since chemicals used in the nuclear power plant may become radioactive wastes. After film formation, organic acid is decomposed into carbon dioxide and water. Representative organic acids that can be decomposed easily are formic acid, malonic acid, di-glycolic acid, and oxalic acid. The samples B and C of FIG. 1 use formic acid to form ferrite films. For example, the sample B uses formic acid solution that contains only iron (II) ions to form ferrite film. The sample C uses formic acid solution in which the concentration of nickel ions is half of the concentration of iron ions so that the iron-nickel ratio of the solution may be equal to the chemical composition ratio of nickel-ferrite.

FIG. 2 shows the result of analysis of a ferrite film formed on the surface of the sample C by the Laser Raman method. As seen in FIG. 2, the Raman peak of the ferrite film of the sample C is a little shifted from the standard nickel-ferrite Raman peak, but it can be assumed that the analyzed film is a nickel-ferrite film judging from the composition. It is assumed that this positional deviation of the peak is caused by non-stoichiometric effect of nickel ferrite, which contains a little magnetite.

The method of suppressing corrosion of carbon steel members in accordance with the present invention is preferably applicable to carbon steel members composing the condensate system or feed water system of a power plant but is not limited thereto. For example, the present invention can be applied to a method of suppressing corrosion of wetted carbon steel members in the auxiliary equipment cooling system and the cooling water system that uses a cooling tower. In short, the present invention can be applied when carbon steel members are wet with water.

A ferrite film formation apparatus for the method of suppressing corrosion of carbon steel members in accordance with the present invention comprises a surge tank for storing processing solution, a circulation pump for sucking the processing solution from the surge tank,

a processing solution supply pipe for introducing the processing solution pressurized by the pump to pipings forming film,

a first chemical tank for storing iron (II) ions to be added to the processing solution that flows through the processing solution supply pipe,

a second chemical tank for storing oxidizing agent to be added to the processing solution that flows through the processing solution supply pipe,

a third chemical tank for storing a pH adjustment agent to adjust pH of the processing solution in the range of 5.5 to 9.0,

a processing solution return pipe for introducing the processing solution from the pipes forming film to the surge tank, and

a heating apparatus for heating the processing solution to a temperature of 60° C. to 100° C.

To attain the second object of the present invention, inventors of the present invention made various studies and researches and found that fine magnetite film can be suppressed taking in of cobalt of radionuclide. The fine magnetite film is formed under a thermal condition at which dissolved oxygen diffuses slower into base metal, for example, under a thermal condition of 100° C. or lower. Below will be explained the result of this consideration.

The samples D and E are prepared. The sample D is a carbon steel piece whose surface is mechanically polished. The sample E is a carbon steel piece whose surface is covered with a ferrite film including magnetite as the main component. This ferrite film was formed at a temperature of 100° C. or lower. These samples D and E are immersed in high-temperature water under the BWR operating condition and then a deposit amount of Co-60 is examined. FIG. 3 shows the examined result. In FIG. 3, the ordinate indicates the relative values of the deposit amount of Co-60 of the samples D and E. FIG. 3 shows the deposit amount of Co-60 to the sample E is suppressed in comparison with the sample D. The sample D is a polished metal piece and had no rust on it before the corrosion test. However, in the actual power plant, the carbon steel members already have rust on their surfaces before the operation of the nuclear power plant is started and are ready to store radionuclide. Consequently, in the carbon steel members of the actual power plant, the deposit amount of the radionuclide on the carbon steel members increase after the operation of the nuclear power plant. Therefore, it is assumed that a carbon steel member without a ferrite film takes in Co-60 more easily than a carbon steel member covered with a ferrite film. A method for forming a ferrite film including the magnetite as the main component is disclosed in above JP63-15990B.

As one of methods of forming ferrite films on the surface of carbon steel members composing a nuclear power plant by using a chemical that does not contain chlorine, the inventors invented a method of suppressing deposit of radionuclide on the carbon steel members. This method is characterized by preparing a first chemical including iron (II) ions, a second chemical for oxidizing the iron (II) ions to iron (III) ion, and a third chemical for adjusting pH of processing solution, mixing the first and second chemicals under a temperature condition of ordinary temperature to 100° C., preparing a processing solution whose pH is adjusted in the range of pH 5.5 to pH 9.0 by mixing the resulting mixture and the third chemical, and forming a ferrite film on the surface of carbon steel members by using this processing solution.

The first chemical including iron (II) ions is prepared by dissolving iron into organic aid or carbonic acid. Since used chemicals become radioactive wastes in the nuclear power plant, the first chemical should preferably be an organic acid that can be easily decomposed into carbon dioxide and water. Representative organic acids that can be decomposed easily are formic acid, malonic acid, di-glycolic acid, oxalic acid and the like. As the result of the film formation test shown in FIG. 4, the inventors found that these organic acids can form the magnetite film on the surface of carbon steel members, and more particularly that formic acid has the greater effect on film formation speed and uniformity.

Immediately after the chemicals are mixed up, the processing solution starts forming fine magnetite particles in the solution even when there is no object to be film-formed. Therefore, the chemicals must be mixed up just before the film formation starts. To determine the order of mixing up the chemicals to effectively form magnetite films, the inventors prepared two mixing orders; adding first an oxidizing agent and then a pH adjustment agent to the processing solution including the iron (II) ions, and adding first a pH adjustment agent and then an oxidizing agent to this processing solution, and checked the resulting magnetite films. FIG. 5 shows the result of film formation. When the oxidizing agent is added after the pH adjustment agent, the resulting ferrite film contains greater magnetite particles, and is dispersed unevenly. Contrarily, when the pH adjustment agent is added after the oxidizing agent, the resulting ferrite film closely contains uniform magnetite particles and the rate of film formation is great.

The chemicals of iron (II) ions, oxidizing agent, and pH adjustment agent must be added into the circulation system in that order from the upstream side of system. Particularly, the adding point of pH adjustment agent should be placed in the downstream of the circulation pump and just in the upstream of the object forming the film. This is to avoid formation of unwanted ferrite film in the temporary piping.

To perform both chemical decontamination and ferrite film formation, it is possible to connect an oxidizing agent tank and a reducing agent tank for chemical decontamination respectively to the solution supplying pipe that leads to the piping that the film is to be formed.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant should preferably remove contaminants from the surface of the carbon steel member before forming ferrite films, which is a pre-treating process of the ferrite formation.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant should preferably perform chemical decontamination which contains at least one reducing removal step to remove contaminants.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant should place preferably a position to add the third chemical in the reactor containment vessel.

Another invention suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant should preferably use a solution in which iron (II) ion in formic acid as the first chemical is dissolved.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant should preferably perform ferrite film formation in a time period between the end of decontamination removal and the start of the operation of the nuclear power plant.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant can comprise the steps of:

preparing a first chemical that contains iron (II) ions, a second chemical that contains iron (III) ion, and a third chemical that adjusts pH of processing solution;

obtaining a processing solution whose pH is adjusted in the range of pH 5.5 to pH 9.0 by mixing the first and second chemicals under a temperature condition of ordinary temperature to 100° C., and mixing the resulting mixture and the third chemical; and

forming ferrite films on the surface of carbon steel members by using the processing solution.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant can preferably use formic acid as the organic acid.

A ferrite film formation apparatus for forming ferrite films on the surface of carbon steel members composing a nuclear power plant to suppress corrosion of carbon steel members is characterized by comprising a surge tank for storing processing solution, a circulation pump for sucking the processing solution from the surge tank,

an outward pipe for supplying the processing solution exhausted from the circulation pump to piping to form film therein,

a first chemical tank for storing iron ions to be added to the processing solution,

a second chemical tank for storing oxidizing agent to be added to the processing solution,

a third chemical tank for storing a pH adjustment agent to adjust pH of the processing solution that is a mixture of chemicals from the first and second chemical tanks in the range of 5.5 to 9.0,

a homeward pipe for introducing the processing solution from the piping being formed the film into the surge tank, and

a heating apparatus for heating the processing solution to a temperature of 60° C. to 100° C.,

wherein the position of adding the third chemical is provided in the reactor containment vessel.

Another invention of suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant is preferably applied to members composing the reactor water purifying system or residual heat removal system of the BWR plant. However, the deposit suppressing method is not limited thereto. For example, the method is also applicable as a method of suppressing deposit of radionuclide on the carbon steel members that are in contact with reactor water in a CANDU (Canada Deuterium Uranium) type heavy-water reactor

In accordance with the present invention that attains the first object, it is possible to suppress corrosion of carbon steel members composing the nuclear power plant.

In accordance with the present invention that attains the second object, it is possible to effectively suppress deposit of radionuclide on carbon steel members composing a nuclear power plant and to protect persons who are working on periodic inspection of the nuclear power plant against exposure to radiations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing the result of an experiment that tests the effect of suppressing corrosion of carbon steel members.

FIG. 2 is an explanatory drawing showing the result of a laser raman analysis of the nickel-ferrite films.

FIG. 3 is an explanatory drawing showing the result of an experiment that places stainless steel samples covered with a ferrite film containing magnetite as the main component in hot-temperature water under the BWR operating condition and measures their deposit amount of Co-60.

FIG. 4 is an explanatory drawing showing the relationship of kinds of organic acids and formation rate of ferrite films.

FIG. 5 is an explanatory drawing showing the relationship of the adding order of chemicals and rate of film formation.

FIG. 6 is a flow chart showing a method for suppressing corrosion of carbon steel of an embodiment 1 which is one preferred embodiment of the present invention.

FIG. 7 is an explanatory drawing showing the connection of a film formation apparatus to the feed water pipe of the BWR plant when the method of suppressing corrosion of carbon steel members of FIG. 6 is applied to the feed water of the BWR plant.

FIG. 8 is a detailed structural diagram showing the film formation apparatus of FIG. 7.

FIG. 9 is a flow chart showing a method for suppressing corrosion of carbon steel of embodiment 2 which is another embodiment of the present invention.

FIG. 10 is a detailed structural diagram showing the film formation apparatus of FIG. 9 which is used for the method of suppressing corrosion of carbon steel members.

FIG. 11 is an explanatory drawing showing the connection of a film formation apparatus to the feed water pipe in the secondary system of the PWR plant when the method of suppressing corrosion of carbon steel members of embodiment 3 which is another embodiment of the present invention is applied.

FIG. 12 is an explanatory drawing showing the connection of a film formation apparatus to the feed water pipe of a thermal power plant when the method of suppressing corrosion of carbon steel members of embodiment 4 which is still another embodiment of the present invention is applied.

FIG. 13 is a detailed schematic system diagram showing another film formation apparatus to which the present invention is applied.

FIG. 14 is a flow chart showing a method for suppressing deposit of radionuclide of embodiment 6 which is another embodiment of the present invention.

FIG. 15 is an explanatory drawing showing the connection of a film formation apparatus to the pipe of a reactor water purifying system of a nuclear power plant when the deposit suppressing method of FIG. 14 is applied to the pipe of the reactor water purifying system of the plant.

FIG. 16 is an explanatory drawing showing the connection of a film formation apparatus to the pipe of a residual heat removal system of a nuclear power plant when the deposit suppressing method of FIG. 14 is applied to the pipe of the residual heat removal system of the plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Below will be explained some embodiments of the present invention concerning to the method of suppressing corrosion of carbon steel members that constitute a nuclear power plant.

Embodiment 1

FIG. 6 is a flow chart showing a method for suppressing corrosion of carbon steel of an embodiment 1 which is one preferred embodiment of the present invention. FIG. 7 is an explanatory drawing showing the connection of a film formation apparatus to the feed water pipe of the BWR plant when the method of suppressing corrosion of carbon steel members of FIG. 6 is applied to the feed water of the BWR plant. FIG. 8 is a detailed structural diagram showing the film formation apparatus of FIG. 7.

As shown in FIG. 7, the BWR plant comprises a reactor 1 having a reactor pressure vessel loading a plurality of fuel assemblies filled with nuclear fuel materials which generates nuclear fission, a main steam pipe 2 connected to the reactor 1, a steam turbine 3 connected to the main steam pipe 2, and a condenser 4 connected to the steam outlet of steam turbine 3. Steam generated in the reactor is supplied to the steam turbine 3. The steam exhausted from the steam turbine 3 is condensed by condenser 4. The condensate which is water is pressurized up by a condensate pump 5 and a feed water pump 7 and supplied as feed water to the reactor 1 through a feed water pipe 10 of feed water system. The feed water pipe 10 connected to the condenser 4 connects the condensate pump 5, a condensate demineralizer 6 to purify condensate, the feed water pump 7, low pressure feed water heaters 8, and high pressure feed water heaters 9 in that order. The low pressure feed water heater 8 and the high pressure feed water heater 9 use steam extracted from steam turbine 3 as their heat sources.

The BWR plant provides with a plurality of re-circulation systems supplied cooling water (reactor water) to the core in which a plurality of fuel assemblies are load. The re-circulation systems respectively contain a re-circulation pipe 22 with re-circulation pump 21. The reactor water surrounding the core in reactor 1 is pressurized by re-circulation pump 21 and jetted in a jet pump (not shown) arranged in the reactor 1 through the re-circulation pipe 22. The reactor water exhausted from the jet pump jets is introduced into the core.

The reactor water purifying system to purify reactor water in the reactor 1 is provided with a purifier pipe 20 one end of which is connected to the re-circulation pipe 22 and the other end is connected to the feed water pipe 10. This reactor water purifying system, further, is equipped with a purifying system pump 24, a regenerative heat exchanger 25, non-regenerative heat exchanger 26, and reactor water purifying apparatus 27 which are provided with the purifier pipe 20. Part of the reactor water flowing through the re-circulation pipe 22 is introduced into the purifier pipe 20 by the purifying system pump 24. The reactor water is cooled by the regenerative heat exchanger 25 and the non-regenerative heat exchanger 26, purified by the reactor water purifying apparatus 27, and heated by the regenerative heat exchanger 25. The reactor water exhausted from the regenerative heat exchanger 25 is supplied to the reactor 1 through the feed water pipe 10 in the downstream side of the high pressure feed water heater 9.

As shown in FIG. 7, a film formation apparatus 30 being used in the method for suppressing corrosion of carbon steel members of the present embodiment is connected to the feed water pipe 10 so as to bypass the feed water pump 7, the low pressure feed water heater 8, and the high pressure feed water heater 9. In other words, when reactor 1 was shut down, the heaters 8 and 9 are bypassed for example by opening the bonnet of valve 23 which is provided at the exit of condensate demineralizer 6, closing the opening of feed water pipe 10 in the demineralizer side, connecting one end of a valve 34 in a processing solution pipe 35 of the film formation apparatus 30 to the opening end of the feed water pipe 10 in the feed water pump side with the flange of the valve 23, disconnecting the feed water pipe 10 (for example, drain pipe or sampling pipe) in the downstream side of the high pressure feed water heater 9, and connecting the valve 47 of the processing solution pipe 35 to the disconnected opening. Although film formation apparatus 30 is connected to the feed water pipe in this embodiment, the connection is not limited thereto. Film formation apparatus 30 can be connected to any wetted portion of the carbon steel member composing the nuclear power plant such as those in the condensate system, auxiliary equipment cooling water system, and cooling water system that uses a cooling tower.

The film formation apparatus 30 is so constructed as to work also for chemical decontamination. For example, the film formation apparatus 30 are equipped with a surge tank 31 which stores water for film formation, and a circulation pump 32 to pressurize water in the surge tank 31 as shown in FIG. 8. The circulation pump 32 supplies the water from the surge tank 31 to one end of the feed water pipe 10 through the processing solution pipe 35 with which the valves 33 and 34 are provided. A chemical tank 40 is connected to the processing solution pipe 35 through a valve 38 and an injection pump 39. The chemical tank 40 stores hydrazine as a chemical to adjust pH of the processing solution. Further, a flow path is provided from the discharge side of the circulation pump 32 to the surge tank 31 through a valve 36 and an ejector 37. The ejector 37 is equipped with a hopper that supplies permanganic acid to oxidize and dissolve contaminants in pipes or oxalic acid to reduce and dissolve contaminants in pipes.

A chemical tank 45 is connected to the processing solution pipe 35 via an injection pump 43 and a valve 41. A chemical tank 46 is connected to the processing solution pipe 35 via an injection pump 44 and a valve 42. The chemical tanks 45 and 46 store chemicals for formation of ferrite films. For example, the chemical tank 45 stores an iron (II) ion solution prepared by dissolving iron in formic acid. The chemicals capable of dissolving iron are not limited to formic acid and can be organic acid or carbonic acid that can be pair anions of iron (II) ions. The chemical tank 46 stores hydrogen peroxide as an oxidizing agent used for film formation.

Meanwhile, the processing solution supplied from the circulation pump 32 to one end of feed water pipe 10, flows through the feed water pipe 10 (the feed water pump 7, the low pressure feed water heater 8, and the high pressure feed water heater 9), and is returned to the valve 47 from the other end of the feed water pipe 10. Then, the processing solution is returned to the surge tank 31 through the processing solution pipe 35 having a circulation pump 48, a valve 49, a heater 53, and valves 55, 56, and 57. A valve 50 and a filter 51 are connected to the processing solution pipe 35 so as to bypass the valve 49. A cooler 58 and a valve 59 are connected to the processing solution pipe 35 in parallel with the heater 53 and the valve 55. A cation exchange resin column 60 (filled with, for example, cation exchange resin) and a valve 61 are connected to the processing solution pipe 35 in parallel with the valve 56. A set of mixed-bed resin column 62 is connected to the processing solution pipe 35 in parallel with the valve 56. A decomposition apparatus 64 and valve 65 are connected to the processing solution pipe 35 in parallel with the valve 57.

The decomposition apparatus 64 is connected to the discharge side of injection pump 44 being connected to chemical tank 46 through a valve 54. In this configuration, hydrogen peroxide solution in the chemical tank 46 can be introduced to the decomposition apparatus 64. The present embodiment uses hydrogen peroxide as oxidizing agent for both the formation of the ferrite films and the decomposition of the processing solution. Therefore, the chemical tank 46 and injection pump 44 can be used for both the formation of the ferrite films and the decomposition of the processing solution. Thus, the present embodiment can simplify the facility. However, if pipes are lengthened because of insufficient installation spaces, the decomposition apparatus for the formation of the ferrite films and the decomposition apparatus for the decomposition of the processing solution can be provided separately.

The valve 42 to inject the oxidizing agent is connected to the processing solution pipe 35 in the downstream side of the valve 41 injecting iron (II) ions. Further, the valve 42 is connected to the processing solution pipe 35 in the upstream side of the valve 38 injecting pH adjustment agent. It is preferable that the valve 38 is connected to the processing solution pipe 35 as close to an object that the ferrite film is to be formed as possible in the downstream side of the valve 42. It is preferable that the film formation apparatus 30 is constitute so as to feed the processing solution from the circulation pump 48 to filter 51 (see FIG. 10) after the ferrite films have been formed.

Inert gas such as nitrogen or argon should preferably be spouted out in the aqueous solution in the chemical tank 45 and the surge tank 31 for storing the chemicals including the iron (II) ion to remove oxygen being included the aqueous solution. Decomposition apparatus 64 can decompose organic acid being used as the pair anions of iron (II) ions and hydrazine being used as a pH adjustment agent. In short, chemicals used as pair anions of iron (II) ions can be organic acids that can be decomposed into water and carbon dioxide to reduce the waste amount or carbonic acid that can be exhausted as gas and does not increase the waste amount. To suppress the amount of chemicals being used, it is preferable to separate and remove excessive reaction products, and to recover and re-use unreacted chemicals being left in the processing solution.

Referring to the flow chart of FIG. 6, below will be explained a procedure to form the ferrite films by using film formation apparatus 30. First, the film formation apparatus 30 is connected to a piping system including the carbon steel members that is the object that the ferrite film is to be formed on the surface thereof (step S1). For example, the film formation apparatus 30 is connected to the feed water pipe 10 that is made of carbon steel member as shown FIGS. 7 and 8.

Corrosive products such as oxide films on the internal surface of the feed water pipe 10 is chemically decontaminated and removed by film formation apparatus 30 (step S2). It is preferable to perform chemical decontamination in the method of suppressing corrosion of carbon steel members in accordance with the present embodiment. However, this chemical decontamination can be omitted if the surface of carbon steel members on which a ferrite film is formed is exposed before the ferrite film is formed. Mechanical decontamination such as grinding can be applied to remove contaminants instead of the chemical decontamination.

The chemical decontamination of Step S2 is a well-known method that is applied to chemically decontaminate in the recirculation system, however, it will be briefly explained below. First, the valves 33, 34, 47, 49, 55, 56, and 57 are opened, under the condition that the other valves are closed. The circulation pumps 32 and 48 are started to circulate water from the surge tank 31 through the feed water pipe 10 that is target of the chemical decontamination. Further, the temperature of the circulating water is raised to about 90° C. by heater 53. After the heating, the valve 36 is opened to supply potassium permanganate (oxidizing agent) of a predetermined amount from a hopper connected to ejector 37 into the surge tank 31. The potassium permanganate is dissolved in the water in the surge tank 31. The chemical decontamination solution (oxidizing decontamination solution) including the dissolved potassium permanganate is introduced into the feed water pipe 10 through the processing solution pipe 35 and oxidizes and dissolves part of the corrosive products such as oxide films from the feed water pipe.

After the above chemical decontamination using the oxidizing decontamination solution is completed, oxalic acid is supply from the above hopper into the surge tank 31 to decompose permanganate ions that is left in the oxidizing decontamination solution. Step S2 can be omitted for the feed water system since the feed water system does not contain so much oxide such as chromium to be oxidized and dissolved. Oxalic acid (reducing agent) is further added to the chemical decontamination solution, in which the permanganate ions were removed, to reduce and dissolve the corrosive products on inner surface of the feed water pipe 10. To adjust pH of the chemical decontamination solution (reducing decontamination solution) including the oxalic acid, the valve 38 is opened and the injection pump 39 is started to inject the hydrazine from the chemical tank 40 into the reducing decontamination solution flowing in the processing solution pipe 35. After the oxalic acid and the hydrazine are injected in the reducing decontamination solution, the valve 61 is opened and the degree of the opening of the valve 56 is adjusted. Therefore, part of the reducing decontamination solution is introduced to the ion exchange column 60. Metal cations (ex. Iron (II) ions) eluted from the feed water pipe 10 into the reducing decontamination solution are adsorbed by cation exchange resin in the ion exchange column 60 and removed from the reducing decontamination solution. The aforesaid oxidizing decontamination solution is also one of chemical decontaminators.

After the above chemical decontamination by reducing dissolution is completed, the valve 65 is opened and the degree of the opening of the valve 65 is adjusted. At the same time, the degree of the opening of the valve 57 is adjusted to reduce its opening. With this, part of the reducing decontamination solution is supplied to the decomposition apparatus 64. The valve 54 is opened and the injection pump 44 is rotated at the same time. The hydrogen peroxide in the chemical tank 46 is added to the reducing decontamination solution being introduced into the decomposition apparatus 64 and decomposes oxalic acid and hydrazine in the reducing decontaminator.

To remove impurities from the chemical decontamination solution after the oxalic acid and hydrazine are decomposed, heater 52 is turned off and valve 55 is closed. At the same time, the valve 59 of cooler 58 is opened to supply the decontamination solution to cooler 58 and the temperature of the decontaminator is reduced. After the decontamination solution is cooled down enough to be supplied to the mixed bed resin column 62, the valve 61 is closed and the valve 63 is opened to supply the whole flow rate of the decontamination solution to the mixed bed resin column 62. Therefore, residual impurities are removed from the decontamination solution.

A series of the operations that are oxidizing dissolution, decomposition of the oxidizing agent, reducing dissolution, decomposition of the reducing agent, and purification can dissolve and remove the corrosive products including the oxidized films from the carbon steel members being the target to be decontaminated.

In this way, after corrosive products including oxidized films of the carbon steel members are removed, a ferrite film forming processing in accordance with the present embodiment is performed. After finishing of the purification operation of Step S2, the valve 50 is opened and the valve 49 is closed to introduce the processing solution to filter 51. The processing solution passing through filter 51 is heated to a set temperature by heater 53 (Step S3). If the processing solution contains fine solids, ferrite films are also formed on the surface of the solids while the ferrite film forming processing is performed. Therefore, it is possible to prevent waste of chemicals by supplying the processing solution to filter 51 and removing such solids. However, this supply of processing solution to filter 51 is not appropriate because hydroxide (ex. iron hydroxide) of high concentration is removed by filter 51 and because the pressure loss of the filter 51 increases. The hydroxide is formed based on iron (II) ion, which is generated by the dissolution of the corrosive products, of high concentration in the processing solution. The valve 56 is opened and the valve 63 is closed to stop supply of the processing solution for purification to the mixed bed resin column 62.

Here, the temperature of the processing solution is preferably about 75° C. but not limited thereto. Here, the most important thing is that the temperature is high enough to form ferrite films having so fine and strong structure (including crystal structures) to protect carbon steel members against corrosion during the operation of the reactor. Therefore, the preferable temperature condition should be a temperature of lower than the maximum operating temperature of the feed water system or a temperature of at least 200° C. or lower. Further, the low temperature limit can be ordinary temperature, but preferably be 60° C. or higher at which the practical film formation speed is obtained. When the temperature is 100° C. or higher, the facility must be pressurized to suppress boiling of the processing solution. Therefore, the temporary facility is required resistance of pressure and increases the construction cost. Thus, the temperature to form the fine ferrite films should preferably be 100° C. or lower.

To form the ferrite films, iron (II) ions must be adsorbed to the surface of an object that the ferrite film is to be formed. However, if the processing solution contains dissolved oxygen, iron (II) ions are oxidized to iron (III) ions by dissolved oxygen according to a reaction expressed by Formula (1). Since the iron (III) ions are lower in solubility than the iron (II) ions, the iron (III) ions are precipitated out of the processing solution as iron hydroxide according to a reaction expressed by Formula (2), so that the formation of the ferrite films is blocked by the precipitation of iron hydroxide. Therefore, it is preferable to spout out the inert gas in the processing solution or vacuum deaeration to remove dissolved oxygen from the processing solution.

4Fe²⁺+O₂+2H₂O→4Fe³⁺+4OH⁻  (1)

Fe³⁺+3OH⁻→Fe(OH)₃  (2)

A chemical including the iron (II) ions from the chemical tank 45 is injected into the processing solution by opening the valve 41 and rotating the injection pump 43 when the temperature of the processing solution reaches the set temperature (step S4). The iron (II) ions in the chemical are adsorbed to the surface to the carbon steel members that the ferrite film is to be formed. The chemical contains the iron (II) ion that was prepared by dissolving iron in formic acid. Then, in order to form the ferrite films by oxidizing the iron (II) ions adsorbed on the surface of the carbon steel members, the oxidizing agent from chemical tank is injected into the processing solution by opening the valve 42 and rotating the injection pump 44 into the processing solution (step S5). Hydrogen peroxide used to decompose the chemical decontamination solution is also used as the oxidizing agent. However, the oxidizing agent can use a solution dissolving ozone or oxygen. Finally, the pH adjustment agent from chemical tank 40 is injected into the processing solution by opening the valve 38 and rotating the injection pump 39 (Step S6). The pH adjustment agent is for example hydrazine. In treatment of Step S6, pH of the processing solution is adjusted to 5.5 to 9.0 which is the condition of starting reaction to form the ferrite films. Consequently, the reaction to form the ferrite films advances. Thus, Steps S4 to S5 form oxidize films being the ferrite films (hereinafter called magnetite films) including magnetite as the main component on the wetted surface of the carbon steel members.

Steps S4, S5, and S6 should preferably be conducted continuously. More specifically, it is preferable to start injection of the oxidizing agent when the processing solution reaches the oxidizing agent injection point after the iron (II) ions are injected into the processing solution. Further, it is preferable to start injection of the pH adjustment agent immediately when the processing solution including the iron (II) ions and the oxidizing agent reaches the pH adjustment agent injection point. If the processing solution including only the iron (II) ions is circulated through the processing solution pipe 35, the iron (II) ions may be oxidized by dissolved oxygen being left in the processing solution. This may cause losses of the chemicals due to unwanted reaction and inhibition of the regular reaction.

When the oxidizing agent is added to the processing solution including the iron (II) ions, the oxidative reaction of the iron (II) ions starts and the ratio of the iron (II) ions to iron (III) ions in the processing solution becomes fit for the film formation reaction. However, the film formation reaction does not advance because the processing solution is acidic in this status. By adding the pH adjustment agent to the processing solution, the film formation reactions start. Therefore, to prevent formation of unwanted films on the inner surface of the temporary pipe, the point of injecting the pH adjustment agent should preferably be close to object that the ferrite film is to be formed and a point at which the temporary facility is connected to the structure members of the BWR plant.

As already explained for Steps S4 to S6, chemicals are injected in the order of the iron (II) ions, the oxidizing agent, and the pH adjustment agent. The order of the oxidizing agent, the iron (II) ions and the pH adjustment agent can be reversed. If hydrogen peroxide (as the oxidizing agent) is added first, however, part of hydrogen peroxide may be used in vain because hydrogen peroxide may be easily decomposed on the surface of the hot metal. Further, when chemicals are injected in the order of the iron (II) ions, the pH adjustment agent and the oxidizing agent, the ferrite films can be formed but magnetite particles in the formed ferrite film may be comparatively greater. In summary, when the chemicals are injected in the order that is described in Steps S4, S5, and S6, the chemicals are used effectively and fine magnetite films can be formed.

After the film formation process is finished, a waste solution treatment process is performed (Steps S7 and S8). Contrarily, when the film formation process is not finished, the processes of the Step S4 to S7 are performed again by adding continuously the chemical to the processing solution to form a magnetite film of a desired thickness.

When the film formation process is finished, the processing solution used to form the magnetite films still contains formic acid and hydrazine. So the waste solution treatment process of Step S8 is performed. That is, the formic acid and hydrazine must be removed from the processing solution before the processing solution is exhausted from the film formation apparatus 30. If the remainders in the processing solution are treated by cation exchange resin in the cation exchange resin column 60, wastes (used cation exchange resins) from the cation exchange resin column 60 will increase. To avoid this, the waste solution treatment process of Step S8 should preferably decompose formic acid in the processing solution into carbon dioxide and water and hydrazine into nitrogen and water by the decomposition apparatus 64 that was used to decompose the decontamination solution. This can reduce the load of the cation exchange resin column 60 and the amount of waste from the cation exchange resin column 60.

The decomposition process of formic acid and hydrazine is similar to decomposition of oxalic acid. The decomposition process comprises steps of adjusting the degree of the openings of valves 65 and 57 to supply part of the processing solution to decomposition apparatus 64 and supplying hydrogen peroxide from the chemical tank 46 into the processing solution being introduced into decomposition apparatus 64. Formic acid and hydrazine are decomposed in the decomposition apparatus 64.

In accordance with the present embodiment, the magnetite film is formed on the surface of the object (carbon steel members) the ferrite film is to be formed while suppressing the amount of waste from the cation exchange resin column 60. The magnetite film can protect the wetted surfaces against corrosion during the operation of the nuclear power plant. Consequently, the present embodiment can omit addition of oxygen to feed water that has been performed to suppress corrosion. Further, the present embodiment will not spoil the soundness (for example, corrosion resistance) of the structural materials-composing the nuclear power plant because chemical containing chlorine is not used for the film formation.

Embodiment 2

Embodiment 2 is different from Embodiment 1 in that Embodiment 2 forms nickel ferrite films and Embodiment 1 forms the magnetite films.

Aforesaid Embodiment 1 describes the procedure and the apparatus to form the magnetite film on the inner surface of the carbon steel pipes of the feed water system. In the result (see FIG. 1) of a test of dipping samples in pure water at ordinary temperature, both magnetite and nickel ferrite films can obtain almost the same corrosion suppressing effect. However, in the actual service environment of the feed water system which uses pure water at high temperature, the nickel ferrite films have longer corrosion suppressing effect than the magnetite films because the solubility of the nickel ferrite is less than that of the magnetite. The embodiment 2 forms the nickel ferrite film on the surface of wetted surfaces of the carbon steel members composing the nuclear power plant.

FIG. 9 shows a method of suppressing corrosion of carbon steel members which is another embodiment of the present invention, and specifically a flow chart of Embodiment 2 for forming nickel ferrite films. FIG. 10 shows a detailed structural diagram of the film formation apparatus being used the formation of the nickel ferrite films.

As seen from FIG. 9, Embodiment 2 is different from Embodiment 1 in that Step S4′ of injecting a chemical including nickel ions to the processing solution is added to the procedure of forming the ferrite films. The process of Step S4′ is carried out after Step S4 of adding the chemicals including the iron (II) ions to the processing solution and before Step S5 of adding hydrogen peroxide to the processing solution. To accomplish Step S4′, the film formation apparatus 30A used by the present embodiment is additionally equipped with a chemical tank 68, an injection pump 67, and a valve 66. The other procedure and apparatus of Embodiment 2 are basically the same as those of Embodiment 1.

In Embodiment 2, the chemical tank 68 for storing the nickel ions is provided separately from the chemical tank 45 for storing the iron (II) ions. This is to avoid the formation of the precipitation in the solution including the iron (II) ion. When the solution including the iron (II) ion contains formic acid and nickel ions, the precipitation is formed in this solution. Therefore, it is preferable to use a nickel carbonate solution as a solution including the nickel ions. If a solution that can dissolve both iron (II) ions and nickel ions is used, chemical tank for storing the solution including the iron (II) ions and chemical tank for storing the solution including the nickel ion as in Embodiment 2 need not be separated. Embodiment 2 can form the nickel ferrite films by using the same procedure and apparatus as those of Embodiment 1 by adding the nickel ions to the processing solution including the iron (II) ions.

In accordance with the present embodiment, the corrosion of the carbon steel members composing the nuclear power plant can be suppressed for a comparative long period because the nickel ferrite films formed on the surface of the carbon steel members are less soluble to water than the magnetite films.

Embodiment 3

Embodiment 3 is different from Embodiment 1 in that Embodiment 3 connects the film formation apparatus 30 to the feed water pipe 10 of the secondary system of a PWR plant but Embodiment 1 connects the film formation apparatus 30 to the feed water pipe 10 of the BWR plant. FIG. 11 is a structural diagram showing Embodiment 3 that connects the film formation apparatus 30 to the feed water pipe in the secondary system of the PWR plant. FIG. 11 shows only the secondary system including a steam generator 69 but does not contain the configuration of a primary system of the PWR plant.

Embodiment 3 is different from Embodiment 1 (for example, FIG. 7) in that a deaerator 70 is provided with the feed water pipe 10 between the low pressure feed water heater 8 and the high pressure feed water heater 9. The deaerator 70 is used to remove gas components from the feed water. Embodiment 3 is similar to Embodiments 1 and 2 as for a method of connecting the film formation apparatus 30 to the feed water pipe 10 and the film formation procedure. The secondary system of the PWR plant generally uses ammonia, hydrazine, or other chemical to suppress corrosion of carbon steel members. In Embodiment 3, however, since the film formation apparatus 30 performs the film formation process shown in FIG. 1, the treatment of such chemicals as ammonia, and the like is not required. Therefore, the present embodiment can accomplish environment-friendly plant operations and reduce the running cost of apparatus.

Embodiment 4

Embodiment 4 is different from Embodiment 3 in that Embodiment 4 connects the film formation apparatus 30 to the feed water pipe 10 of a thermal power plant but Embodiment 3 connects the film formation apparatus 30 to the feed water pipe of the secondary system of the PWR plant. FIG. 12 shows a thermal power plant in which a film formation apparatus is connected to the feed water pipe of thereof. As shown in FIG. 12, Embodiment 4 is different from Embodiment 3 in that a boiler 71 is used in place of the steam generator 69.

Embodiment 4 is similar to Embodiments 1 and 3 as for a method of connecting the film formation apparatus 30 to the feed water pipe 10 and the ferrite film formation process. Like the secondary system of a PWR plant, the feed water pipe 10 of the thermal power plant generally uses ammonia, hydrazine, or other chemical to suppress corrosion of the carbon steel members. In Embodiment 4, however, since the ferrite film formation is performed by the film formation apparatus 30, the treatment of such chemicals as ammonia, and the like is not required. Therefore, the present embodiment can accomplish environment-friendly plant operations and reduce the running cost of apparatus.

Embodiment 5

Embodiment 5 describes another example of film formation apparatus. FIG. 13 shows a detailed structural diagram of another film formation apparatus to which the present invention is applied. Film formation apparatus 30B shown in FIG. 13 is different from the film formation apparatus 30 of Embodiment 1 (for example, FIG. 8) in that a nitrogen bubbling apparatus 72 is connected to the surge tank 31 and that another nitrogen purging apparatus 73 is connected to the chemical tank 45.

In accordance with the present embodiment, the inside of the surge tank 31 is purged with nitrogen gas to exhaust the dissolved oxygen from the solution in the surge tank 31. This purging can make the processing solution in the surge tank 31 substantially free from oxygen and further suppress oxidization of iron (II) ions in chemical tank 45. Therefore, the production of the iron (III) ion that do not contribute to the formation of the magnetite films in the solution can be suppressed. As the result, the reaction to produce the magnetite films is activated and good magnetite films can be formed.

Embodiment 6

Below will be explained a method for suppressing deposit of radionuclide to carbon steel members composing a nuclear power plant in accordance with Embodiment 6 which is another embodiment of the present invention.

FIG. 14 shows a flow chart of Embodiment 6 which is another preferred embodiment of the present invention to suppress deposit of radionuclide. FIG. 15 is an explanatory drawing to indicate the connection of a film formation apparatus to the pipe of a reactor water purifying system of a nuclear power plant when the deposit suppressing method of FIG. 14 is applied to the pipe of the reactor water purifying system of the plant.

As shown in FIG. 15, Embodiment 6 connects the film formation apparatus 30 to the purifying system pipe 20. In other words, when the reactor 1 was shut down, the bonnet of valve 18 on the purifying system pipe 20 is opened and the re-circulation pipe 22 side of the purifying system pipe 20 is closed. One end of the processing solution pipe 35 (as a temporary pipe) of the film formation apparatus 30 in the valve 34 side is connected to the purifying system pipe 20 in the upstream side of the purifying system pump 24. The bonnet of valve 19 in the upstream side of the regenerative heat exchanger 25 is opened and the purifying system pipe 20 is closed in the side of the regenerative heat exchanger 25. One end of the valve 47 side of processing solution pipe 35 is connected to the purifying system pipe 20 in the downstream side of the purifying system pump 24 by using the flange of the valve 19. The configuration of the BWR plant is the same as that of FIG. 7. The configuration of the film formation apparatus 30 used by the present embodiment is the same as that of FIG. 8.

Referring to FIG. 14, below will be explained a method for suppressing deposit of radionuclide by using the film formation apparatus 30 in accordance with the present embodiment. The film formation apparatus 30 is connected to a piping including the carbon steel members being the object that the ferrite film is to be formed (Step 11). The film formation apparatus 30 is connected to the purifying system pipe 20 as explained above.

Contaminants such as oxide films (that contains radionuclide) formed on the surface of carbon steel members that contacts with water are chemically decontaminated by using film formation apparatus 30 (Step S12). The chemical decontamination of Step S12 is not described in detail here because it is the same as the chemical decontamination of Step S2.

After the contaminants including the oxide films formed the surface of the carbon steel members are removed, the formation process of the ferrite films is performed on inner surface of the carbon steel members composing the purifying system pipe 20. First, the temperature of the processing solution is adjusted to a set temperature by using heater 53 (Step S13). In this case, the valve 50 is opened and the valve 49 is closed to introduce the processing solution to filter 51. Filter 51 removes fine solids from the processing solution. The purpose of the removal of the solids is the same as that of Step S3 of Embodiment 1.

The set temperature is preferably about 100° C. but not limited thereto. The point is, the formed ferrite film must be so fine as not to take in the radionuclide from the reactor water during the operation of the reactor. Therefore, it is preferable that the temperature of the processing solution is at least the designed temperature of the system (for example, the purifying system pipe 20) or lower. The low temperature limit of the processing solution can be ordinary temperature, but preferably be 60° C. or higher at which the practical ferrite film formation speed is obtained. When the set temperature is 100° C. or higher, the facility must be pressurized to suppress boiling of the processing solution. Therefore, the temporary facility must be resistant to the pressure.

To form the ferrite film on the surface of the object (carbon steel members) that the ferrite film is to be formed, it is necessary to let the iron (II) ions be adsorbed to the surface. To reduce the dissolved oxygen in the processing solution to effectively use the iron (II) ions in the processing solution for the formation of the ferrite film, it is preferable to spout out the inert gas in the processing solution or vacuum deaeration to remove dissolved oxygen from the processing solution. This is because, when the processing solution contains too much the dissolved oxygen, the iron (II) ions will be precipitated as iron hydroxide by the reactions of Formulae (1) and (2).

When the temperature of the circulated processing solution reaches the set temperature, the chemical including the iron (II) ions is injected to the processing solution (Step S14). The valve 41 is opened and the injection pump 43 is rotated. Therefore, the chemical prepared by dissolving iron in formic acid, and including the iron (II) ion is injected from the chemical tank 45 into the processing solution. Hydrogen peroxide solution as the oxidizing agent) is injected into the processing solution (Step S15). To form the ferrite films by oxidizing the iron (II) ions adsorbed on the surface of the carbon steel members, the valve 42 is opened, the injection pump 44 is rotated, and the hydrogen peroxide solution is injected from the chemical tank 46 into the processing solution. Then, injects hydrazine being the pH adjustment agent is injected into the processing solution (Step S16). To adjust pH of the processing solution to 5.5 to 9.0 which is the condition of starting reaction to form the ferrite films, the valve 38 is opened and the injection pump 39 is rotated to inject hydrazine to the processing solution from the chemical tank 40. Thus, the ferrite film including magnetite as the main component is formed on the inner surface of the carbon steel members of the purifying system pipe 20.

When it is judged that the formation of the ferrite film including the magnetite as the main component completed in Step S17, the waste solution treatment of Step S18 is carried out. When it is judged that the formation of the ferrite film is not completed in Step S17, the processes of the Step S14 to S17 are performed again by adding continuously the chemical to the processing solution to form a magnetite film of a desired thickness that contains the magnetite as the main component.

When the formation of the ferrite film containing the magnetite as the main component completed, the processing solution contains formic acid and hydrazine. Therefore, before the processing solution is exhausted from the film formation apparatus 30, the waste solution treatment of Step S18 must be performed to remove such impurities. The waste solution treatment of Step S18 is the same as that of Step S8 of Embodiment 1. In this way, it is possible to form the ferrite film including the magnetite as the main component on the surface of the carbon steel members while suppressing the amount of ion exchange resin waste that is, the amount of radioactive waste. Thus, the deposit of radionuclide, for example, radioactive cobalt ions onto the surface of the carbon steel members as seen in FIG. 3 is suppressed. As the result, the present embodiment can suppress the dose rate of pipes of the reactor water purifying system and reduce the exposure dose of the persons who are working on periodic inspection of the nuclear power generation plant.

Further, the present embodiment will not spoil the soundness of the structural materials composing the nuclear power plant because chemical containing chlorine is not used for the film formation.

Embodiment 7

FIG. 16 is an explanatory drawing showing the connection of the film formation apparatus to the pipe of a residual heat removal system of the nuclear power plant when the deposit suppressing method shown in FIG. 14 is applied to the pipe of the residual heat removal system of the plant. The present embodiment is a method for suppressing the deposit of the radionuclide onto the carbon steel members composing the nuclear power plant by connecting the film formation apparatus 30 to the pipe 16 of the residual heat removal system, which is another embodiment of the present invention. The residual heat removal system pipe 16 connected to the re-circulation pipe 22 is equipped with a circulation pump 14 and a heat exchanger 15. When reactor 1 was shut down, the bonnet of the valve 12 provided with the residual heat removal system pipe 16 is opened and the opening of re-circulation pipe 22 side of the pipe 16 is closed. One end of the valve 34 side of the processing solution pipe 35 of the film formation apparatus 30 is connected to the residual heat removal system pipe 16 in the upstream side of the circulation pump 14 by using the flange of the valve 12. The bonnet of the valve 13 provided with the residual heat removal system pipe 16 in the downstream side of the heat exchanger 15 is opened. The opening of the re-circulation pipe 22 side of the pipe 16 is closed. One end of the valve 47 side of the processing solution pipe 35 is connected to the residual heat removal system pipe 16 by using the flange of the valve 13.

The chemical decontamination and the ferrite film formation of the present embodiment are also carried out based on the procedure shown in FIG. 14. The corrosive product (part of which is oxide film) formed on the inner surface of the residual heat removal system pipe 16 scarcely contains chromium because the temperature of the processing solution is lower than the reactor water temperature, that is, usually 150° C. or lower and the base metal of the residual heat removal system pipe 16 is made of carbon steel. Therefore, in the residual heat removal system pipe 16, the oxidation process is not required in the chemical decontamination process. Only a single reducing decontamination process is sufficient. The others of the present embodiment are the same as those of the above-described embodiments.

Since the present embodiment forms the ferrite film on the inner surface of the residual heat removal system pipe 16, the present embodiment suppresses corrosion of the carbon steel members of the pipe 16 during the standby period of the residual heat removal system. Further, the deposit of radionuclide onto the surface of the carbon steel member is suppressed when the cooling water is introduced through the pipe 16. In other words, the present embodiment can keep the dose rate of the residual heat removal system pipe 16 low and reduce the exposure dose of persons who are working on maintenance of the residual heat removal system.

Embodiment 8

Although Embodiments 6 and 7 use the chemical including the iron (II) ions to form the ferrite film, it is possible to use formic acid that does not include the iron (II) ions to form the ferrite film on the surface of the carbon steel members. In other words, when formic acid is supplied to the system and circulated in this system, the base metal of the carbon steel members of the system dissolves and the iron (II) ions dissolve into the solution. Therefore, the iron (II) ions can be used for the formation of the ferrite film.

In accordance with the present embodiment, iron formate including iron (II) ions that can be easily oxidized need not be added from the outside although it takes a long time to prepare the iron (II) ions. 

1. A method for suppressing corrosion of carbon steel, comprising the steps of: adsorbing iron (II) ions on a surface of a carbon steel members composing a nuclear power plant; and forming a ferrite film on said surface of said carbon steel members by oxidizing the absorbed iron (II) ions.
 2. The method for suppressing corrosion of carbon steel according to claim 1, wherein the formation of said ferrite film is performed under a temperature condition from 60° C. to 100° C.
 3. A method for suppressing corrosion of carbon steel, comprising the steps of: contacting a first chemical including iron (II) ions, a second chemical for oxidizing at least one part of the iron (II) ions to iron (III) ion, and a third chemical for adjusting pH of a solution including the first chemical and the second chemical to be 5.5 to 9.0 with a surface of a carbon steel members composing a nuclear power plant, and forming a ferrite film on the surface.
 4. The method for suppressing corrosion of carbon steel according to claim 3, wherein the first chemical contains nickel ions.
 5. The method for suppressing corrosion of carbon steel according to claim 3, wherein the first chemical is formic acid including the iron (II) ions.
 6. The method for suppressing corrosion of carbon steel according to claim 4, wherein the first chemical is formic acid including the iron (II) ions.
 7. The method for suppressing corrosion of carbon steel according to claim 3, wherein the second chemical contains at least one of hydrogen peroxide solution, oxygen, and ozone.
 8. The method for suppressing corrosion of carbon steel according to claim 3, wherein the third chemical is hydrazine.
 9. The method for suppressing corrosion of carbon steel according to claim 3, wherein the formation of the ferrite film is performed under a temperature condition from 60° C. to 100° C.
 10. The method for suppressing corrosion of carbon steel according to claim 3, wherein a gas portion of a tank for storing the first chemical is purged with inert gas.
 11. A method for suppressing corrosion of carbon steel, comprising the steps of: comprising the steps of preparing a second chemical that oxidizes at least one part of iron (II) ions into iron (III) ion, a third chemical that contains nickel ion, contacting a first chemical including iron (II) ions, a second chemical for oxidizing at least one part of the iron (II) ions to iron (III) ion, a third chemical including nickel ion, and a forth chemical for adjusting pH of a solution including the first chemical, the second chemical and the third chemical to be 5.5 to 9.0 with a surface of a carbon steel members composing a nuclear power plant, and forming a ferrite film on the surface.
 12. The method for suppressing corrosion of carbon steel according to claim 11, wherein the first chemical is formic acid including the iron (II) ions.
 13. The method for suppressing corrosion of carbon steel according to claim 11, wherein the second chemical contains at least one of hydrogen peroxide solution, oxygen, and ozone.
 14. The method for suppressing corrosion of carbon steel according to claim 11, wherein the fourth chemical is hydrazine.
 15. The method for suppressing corrosion of carbon steel according to claim 11, wherein the formation of the ferrite film is performed under a temperature condition from 60° C. to 100° C.
 16. The method for suppressing corrosion of carbon steel according to claim 11, wherein the gas portion of a tank for storing the first chemical is purged with inert gas.
 17. A ferrite film formation apparatus for the method of suppressing corrosion of carbon steel members comprising; a surge tank for storing processing solution, a circulation pump for sucking the processing solution from the surge tank; a processing solution supply pipe for supplying the processing solution exhausted from the circulation pump to piping that the film is to be formed; a first chemical tank for storing iron (II) ions to be added to the processing solution that flows through the processing solution supply pipe; a second chemical tank for storing oxidizing agent to be added to the processing solution; a third chemical tank for storing a pH adjustment agent to adjust pH of the processing solution in the range of 5.5 to 9.0; a processing solution return pipe for returning the processing solution from the piping to the surge tank; and a heating apparatus for heating the processing solution to a temperature of 60° C. to 100° C.
 18. The ferrite film formation apparatus according to claim 17, wherein a fourth chemical tank for storing nickel ions is provided.
 19. The ferrite film formation apparatus according to claim 17, wherein the first chemical tank stores a chemical including the iron (II) ions prepared by dissolving iron in formic acid.
 20. A method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant comprising the steps of: preparing a first chemical including iron (II) ions, a second chemical for oxidizing the iron (II) ions into iron (III) ion, and a third chemical for adjusting pH; mixing the first chemical and the second chemical under a temperature condition of ordinary temperature to 100° C.; preparing a processing solution whose pH is adjusted in the range of pH 5.5 to pH 9.0 by mixing the mixture including the first chemical and the second chemical and the third chemical; and forming a ferrite film on the surface of the carbon steel members by using the processing solution.
 21. The method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant according to claim 20, further comprises a step of: removing contaminants including oxide films from the surface of the carbon steel members before forming the ferrite film.
 22. The method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant according to claim 21, wherein removal of the contaminant is performed by a chemical decontamination containing at least one reducing removal.
 23. The method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant according to claim 20, wherein the position to inject the third chemical into the mixture is inside the reactor containment vessel.
 24. The method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant according to claim 20, wherein the first chemical is prepared by solving iron in formic acid.
 25. The method for suppressing deposit of radionuclide onto the carbon steel members composing a nuclear power plant according to claim 21, wherein the formation of the ferrite film is carried out after in a period between the finish of removal of the contaminants and the start of the operation of the nuclear power plant.
 26. A method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant, comprising the steps of preparing a first chemical including iron (II) ions prepared by dissolving iron in organic acid, a second chemical for oxidizing the iron (II) ions into iron (III) ion, and a third chemical for adjusting pH; mixing the first chemical and the second chemical under a temperature condition of ordinary temperature to 100° C.; preparing a processing solution whose pH is adjusted in the range of pH 5.5 to pH 9.0 by mixing the mixture including the first chemical and the second chemical and the third chemical; and forming a ferrite film on the surface of the carbon steel members by using the processing solution.
 27. The method for suppressing deposit of radionuclide onto carbon steel members composing a nuclear power plant according to claim 26, wherein the organic acid is formic acid.
 28. A apparatus for forming ferrite film onto the surface of carbon steel members in the pipings of a nuclear power plant, comprising a surge tank for storing processing solution; a circulation pump for sucking the processing solution from the surge tank; a processing solution supply pipe for supplying the processing solution exhausted from the circulation pump and transfers the processing solution to piping that the film is to be formed; a first chemical tank for storing iron (II) ions to be added to the processing solution that flows through the processing solution supply pipe; a second chemical tank for storing oxidizing agent to be added to the processing solution; a third chemical tank for storing a pH adjustment agent to adjust pH of the processing solution in the range of 5.5 to 9.0, a processing solution return pipe for returning the processing solution from the pipes that require film formation to the surge tank; and a heating apparatus for heating the processing solution to a temperature of 60° C. to 100° C., wherein the position for injecting the third chemical into the processing solution is provided inside the reactor containment vessel. 